Epoxy-, alkyd- and polyurethane-based paints and combinations thereof are commonly used to protect steel structures and/or concrete surfaces against corrosion. In general, polyurethane-based paints require a primer. Their greatest disadvantage, however, is related to health factors. The dry solids content of the polyurethane paints is low, which results in a great amount of volatile substances (solvents) and limits the use of said paints. Even though epoxy paints have several good properties, such as good adhesion to steel, concrete and other substrates, good mechanical strength and chemical resistance, it is necessary to use weather-resistant alkyd, or in particular, polyurethane paint for topcoating, because the epoxy paint chalks, i.e. becomes matt, and often yellows when exposed to weather. The required film thicknesses are within the range of about 150 to 500 μm, which often requires 2 to 4 repeated treatments. Even though the epoxy paints have been developed to achieve thicker films with fewer treatments and a slight improvement in the weatherability has also been achieved, the use of polyurethane as a top coat cannot have been avoided so far. The amount of volatile organic content (VOC) still constitutes a problem for both epoxy and polyurethane paints.
High viscosity has also limited the use of previously known epoxy paints. A low ambient temperature has also imposed restrictions on their use in the open air. Quite recently, reduction in viscosity and improvement in weather resistance of the epoxy paints are achieved by modifying epoxy resins with acrylic monomers and oligomers. At the same time, the use of acrylic monomers and oligomers has considerably shortened drying times and exploitation times of the paints.
Further, silicone paints and silicate paints based on organic and inorganic silicon compounds have been known for a long time. Silicone paints resist well heat, up to 600° C., but they dry poorly and yellow easily. Zinc silicate paints have often been used as a primer for demanding anti-corrosive paintings.
Polysiloxane-based paints, based on silicon and oxygen compounds, have also been developed having excellent weather resistance and particularly improved UV light resistance even as compared with the polyurethanes. However, due to their high price, these polysiloxanes acting as binders are used as a sole binder in paints only in exceptional cases, such as in space technology where requirements are extremely high. The high price has forced paint manufacturers to use silicone and siloxane compounds instead of polysiloxanes for improving and modifying the properties of other binders.
An epoxy-based polysiloxane coating composition useful for corrosion prevention is known from WO 96/16109, the composition including a non-aromatic hydrogenated epoxy resin, a polysiloxane and an organo-oxysilane. As preferred non-aromatic epoxide resins are stated hydrogenated cyclohexane dimethanol and diglycidyl ethers of hydrogenated Bisphenol A-type. When the composition dries, the non-aromatic epoxy resin used forms a straightchain epoxy polymer and not a three-dimensional structure, which is possible in the solution of the present invention. Good weather resistance of the known composition is believed to be specifically due to the linear epoxy polymer. On the other hand, it has been found that the drying times of these compositions are relatively long.
A casting resin composition used for covering optoelectronic components is known from U.S. Pat. No. 5,492,981. Said US patent thus relates to a field of use which is different from the field of use of the present invention. For instance, the hardener type and the curing temperature cited in said patent publication indicate that the known composition in question is not related to paints in any manner whatsoever. The resin component of the casting resin composition according to the US patent contains 5 to 95% by weight of a condensation product which is an epoxy group containing polysiloxane formed of a silanol and an epoxyalkoxysilane at an elevated temperature. According to the publication, the resin component also contains 5 to 95% by weight of epoxy resin. The solution of the present invention, in turn, uses polysiloxane, which does not include epoxy groups, and separately epoxy silane and aliphatic epoxy resin.
The object of the invention is thus to provide a low-viscosity anti-corrosive paint composition which avoids the problems of the corresponding, known epoxy-based paint compositions. The present invention is based on the use of a combination of aliphatic epoxy constituents and specific polysiloxane-containing constituents. Now, it has been found that a paint composition having excellent properties is obtained by using epoxy silane, instead of organo-oxysilane (cf. WO 96/16109), together with an aliphatic epoxy resin. The composition of the invention has an advantage that it has low viscosity, whereby diluting is not generally needed, and that it dries quickly also at low temperatures, even at −5° C. The drying rate and curing at low temperatures of known epoxy compositions can be improved by specific accelerators, which, however, do not act at the same temperatures or they cause other problems, such as more intense yellowing than conventionally and matting (chalking) of a painted surface. These accelerators can also be toxic, for instance, phenol derivatives.
The compositions of the invention have a further advantage that it is possible to apply new coatings over the paints containing said compositions after a long time without that any adhesion or curling problems occur. They need not be coated with a separate weather-resistant topcoat either, film thicknesses can be reduced in many cases and the intervals between maintenance paintings can be prolonged, which in turn reduces costs.